What causes stacked physics boxes sliding and collapsing?

With too few solver iterations the contact constraints between boxes are only partly resolved, leaving residual penetration and slip that accumulates until the stack tips over.

How do I catch this when it only happens for some players?

Capture it automatically from the player's device with the full stack trace, the device and OS, the build, and a breadcrumb trail of what they did. That turns a vague complaint into a specific, reproducible issue you can fix without owning the hardware it happens on.

How to Fix Stacked Boxes Sliding and Collapsing

Quick answer: Increase solver iteration counts, let idle boxes sleep, raise contact friction, and avoid tiny colliders, so resting stacks settle and stay put.

A tower of crates that slowly shears sideways and falls over with nothing touching it is a solver-convergence problem. Resting contacts need enough iterations to fully stabilize. Here is how to fix it.

How to fix it

1. Raise solver iterations

Increase the project's default solver iteration count (and velocity iterations) so resting contacts converge each step, eliminating the slow creep that topples stacks.

2. Allow sleeping

Make sure boxes can go to sleep when nearly still by keeping the sleep threshold reasonable; sleeping bodies stop accumulating numerical drift entirely until disturbed.

3. Increase friction and box size

Give the boxes a higher-friction physics material so they do not slide on each other, and avoid very small or very light colliders, which are numerically harder to stack stably.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.

Most of the time the fix is small. Seeing the failure clearly is the part that actually costs you.